The earliest known ancestors of the mammal lineage that includes everything from humans, to blue whales, to pygmy shrews may have been nocturnal, rodent-like creatures that evolved much earlier than previously thought.
The identity of these ancestors comes from their teeth, which were discovered at cliffs on the coast of England. The discovery of the little creatures, which lived about 145 million years ago, may push the evolution of this mammal group back dozens of millions of years, the researchers said.
"Our fossils are definitely of the oldest eutherians known yet in the fossil record," [researcher Steven] Sweetman said. "They lie at the base of the branch of the tree that led to placentals and, therefore, us."
Sweetman said the smaller Durlstotherium was likely about the size of a mouse, while the larger Durlstodon was probably about the size of a juvenile rat.
These newly discovered fossils are at least 20 million years older than those previously thought to be the earliest known eutherian fossils, Sweetman said. Moreover, the level of evolutionary differences seen in these newfound teeth compared with those of other ancient mammals "implies that eutherians had a very early beginning, and that diversity in eutherians arose much earlier than previously expected," Sweetman said.
[Editor's note: The original study can be found here]
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